Rat retinas were incubated with radioactive leucine, glucosamine, and mannose at various times prior to and following the scheduled onset of light. All three exhibited a stimulation of incorporation into opsin prior to the onset of light when compared to other proteins of the retina. Constant light abolished this specific stimulation of opsin synthesis. These observations suggest that the stimulation of opsin synthesis is a circadian event related to the circadian shedding of rod outer segment membranes that follows the scheduled onset of light.